1. Field of the Invention
The invention relates to powder coating apparatus, more particularly to tribo-electro-gas-dynamic (TEGD) powder charging apparatus. The apparatus comprises a powder gun that is especially useful for coating deep recesses, for example inside surfaces of two piece aluminum beverage cans.
2. Description of the Prior Art
Presently known powder coating apparatus generally employs an externally applied electric field to charge the powders. A first type of electrostatic charging gun uses direct current high-voltage low-amperage electrical power to charge the powder particles as they leave the gun barrel. The voltage used ranges up to and in some cases exceeds 100,000 volts, creating a hazard that can cause fires from arcing and electrical shock to personnel. These types of high voltage power systems are expensive, require safety protection and are subject to failure and resultant system unreliability. Because of the position of the high voltage electrodes, an electrical field is produced between the electrodes and ground, which may be the workpiece, and if not specially protected the gun may be brought too close to the grounded workpiece or other grounded item and cause an arc. It is known that such a field, which is used to transport and guide the powder to the workpiece, greatly limits the ability of the powder to reach into any recesses in the workpiece because of he "Faraday Cage" effect.
A second type of gun in the prior art uses a set of electrodes within the body of the gun. The electrodes are arranged to have the powder pass between them with one electrode at ground and the other at high voltage of about 7,000 to 15,000 volts. This system has a smaller but finite electric field between the end of the gun barrel and the workpieces as compared to the first type of gun. Hence, the second type of gun has fewer Faraday Cage and arcing problems. This type of charging is commonly known as the electro-gas-dynamic (EGD) charging system.
In the prior art triboelectric charging has been attempted, with only limited success. The structure described in U.S. Pat. No. 3,903,321 to Schaad achieved some charging with polytetrafluroethylene as a gun material and with epoxide powders. The present invention may be constructed of steel, aluminum, or plastics as representative examples and is useful in charging not only epoxides but also epoxide-phenolics, acrylics, ionomers, and others.
Electrostatic charging by means of friction, or tribo charging, is thought to occur in several ways. One method is to cause the material to be charged to impinge on another surface by rolling, sliding, or bouncing. Another method may be to entrain the material to be charged in a charged fluid that shares the charge with the material. Another method may be to cause differential accelerations among particles of the material to be charged, causing friction between the particles.
A major problem occurs in the prior art when tribo charging of powder is attempted. Almost any powder will tribo charge if it is given sufficient rubbing contact; however, handling typical powders to get sufficient rubbing contact has undesirable consequences. If the powder is a thermoset curing resin, the material is always curing and the rate of cure is temperature dependent. Excessive rubbing can cause the powder to melt and adhere to the walls of tubes and passages, eventually clogging or fouling these passages. If the powder is propelled at a very high velocity in these passages and if the powder particles impact on objects and surfaces in these passages, the kinetic energy is converted to heat causing the powder particles to adhere and cure. This phenomenon is called "impact fusion" and can soon clog passages.